The development of techniques of gene isolation by DNA-mediated gene transfer in cultured animal cells provides a powerful tool for the analysis of the genetics of cancer. Some human tumors contain genes which can oncogenically transform mouse fibroblasts of the established cell line NIH 3T3. Many of the human oncogenes detected by the NIH 3T3 transfection assay belong to the ras gene family. The human oncogene homologue to the Kirsten sarcoma virus oncogene (human c-K-ras) has been found activated in a broad variety of human tumor cell lines and primary tumors. We have isolated human sequences which span over 45 kilobase pair of the c-K-ras oncogene activated in two lung adenocarcinomas (PR310 and PR371) propagated into nude mice. Based on the conservation of these human sequences in mouse fibroblasts transformants, we conclude that the transforming ability of the human c-K-ras oncogene resides within a 43 to 46 kilobase pair DNA region. Nucleotide sequence analysis in concert with DNA transfection experiments indicate that the PR371 oncogene has been activated by a single base change in the first coding exon which results in the incorporation of cysteine instead of glycine in position 12 of the predicted amino acid sequence. The PR310 oncogene, however, has been activated by a point mutation in codon 61 of the second coding exon which substitutes histidine for glutamine in the c-K-ras gene product. These results indicate that the activation of the c-K-ras oncogene in human lung adenocarcinomas can occur by different mutational events. We plan to continue our studies on the structure of the c-K-ras oncogene from human lung carcinomas and on the mechanisms which regulate the expression of the oncogene in human normal and tumor cells and in mouse and rat fibroblast transformants. (M)